Fields are an added feature of the Fields Expansion. They add Gravity Wells and Slow Zones to galaxies.
Gravity Wells
A Gravity Well is a circular space around a Neutron Star which spins and draws ships into the star.
The star itself damages ships as well as emitting chunks of Radiation which also does damage.
Gravity Wells might have no Radiation, no Neutron Star, or just the gravity effect with neither a star or radiation.
Slow Zones
Slow Zones are an area of space where acceleration is altered and ship dynamics are slowed down.